The present invention relates to a control device for a power steering apparatus used in a vehicle such as an automotive vehicle.
In recent years, automotive vehicles provided with a power steering appratus for lessening the steering force have found increasing usage. According to this power steering apparatus, an oil flow rate or a reation force oil pressure is controlled in accordance witha vehicle speed signal, thereby making it possible to conduct a light and comfortable steering which is high substantially constant from a low speed range to a high speed range. However, for example, if the vehicle speed sensor is out of order at the time of the high speed running, a control under no vehicle speed signal, i.e., equivalent to the halt state will be conducted. This, the steering force rapidly becomes small, resulting in very dangerous running state.
To solve such a problem, as disclosed in the Japanese publication No. 62372/85 (Jikkaisho No. 60-62372), a method has been proposed wherein when it is detected that the number of engine revolutions is above a predetermined value under the condition where the vehicle speed signal fails to be obtained, the vehicle speed signal is switched to a fixed signal which is caused to be produced in a quasi-manner after a certain delay time from the detection time.
However, this method requires a large steering force when the vehicle is steered in the halt condition where the number of engine revolutions is low. Further, with this method, since the steering force suddenly changes before and after a predetermined number of engine revolutions, a preferable running condition cannot be obtained as well.